Unveiling Local Electronic Structure of Lanthanide‐Doped Cs2NaInCl6 Double Perovskites for Realizing Efficient Near‐Infrared Luminescence

Abstract Lanthanide ion (Ln3+)‐doped halide double perovskites (DPs) have evoked tremendous interest due to their unique optical properties. However, Ln3+ ions in these DPs still suffer from weak emissions due to their parity‐forbidden 4f–4f electronic transitions. Herein, the local electronic structure of Ln3+‐doped Cs2NaInCl6 DPs is unveiled. Benefiting from the localized electrons of [YbCl6]3− octahedron in Cs2NaInCl6 DPs, an efficient strategy of Cl−‐Yb3+ charge transfer sensitization is proposed to obtain intense near‐infrared (NIR) luminescence of Ln3+. NIR photoluminescence (PL) quantum yield (QY) up to 39.4% of Yb3+ in Cs2NaInCl6 is achieved, which is more than three orders of magnitude higher than that (0.1%) in the well‐established Cs2AgInCl6 via conventional self‐trapped excitons sensitization. Density functional theory calculation and Bader charge analysis indicate that the [YbCl6]3− octahedron is strongly localized in Cs2NaInCl6:Yb3+, which facilitates the Cl−‐Yb3+ charge transfer process. The Cl−‐Yb3+ charge transfer sensitization mechanism in Cs2NaInCl6:Yb3+ is further verified by temperature‐dependent steady‐state and transient PL spectra. Furthermore, efficient NIR emission of Er3+ with the NIR PLQY of 7.9% via the Cl−‐Yb3+ charge transfer sensitization is realized. These findings provide fundamental insights into the optical manipulation of Ln3+‐doped halide DPs, thus laying a foundation for the future design of efficient NIR‐emitting DPs.

Synthesis of Cs2NaxAg1-xInCl6:Yb 3+ Crystals: High-quality lanthanide-doped Cs2NaxAg1-xInCl6 double perovskites (DPs) were synthesized via a facile one-pot hydrothermal synthesis method. In a typical synthesis, 1.6 mmol of CsCl, 0.8*x mmol of NaCl, 0.8*(1x) mmol of AgCl, 0.8 mmol of InAc3 and 1.2 mmol of YbAc3 were dissolved in 8.0 mL of HCl (10 M) solution in a 20-mL Teflon autoclave. Then the solution was heated at 180 °C for 12 h in a stainless-steel autoclave. The solution was then steadily cooled down to 30 °C at a speed of 3 °C h -1 . Then the crystals were filtered out, washed three times with isopropyl alcohol to remove the solvent from the crystal surface and dried in an oven at 60 °C. Finally, the crystals were ground into powder as a phosphor.
Synthesis of Cs2AgInCl6:Ln 3+ Crystals: The synthesis method of Cs2AgInCl6:xLn 3+ is similar to the above synthesis process of Cs2NaInCl6:xLn 3+ , wherein the only difference is that 0.8 mmol of AgCl was employed instead of NaCl.
Photoluminescence (PL) quantum yield (QY) measurement: A barium sulfate-coated integrating sphere (150 mm in diameter, Edinburgh) was employed as the sample chamber that was mounted by a fiber optic spectrometer (QE65pro, Ocean Optics) with the entry and output port of the sphere located in 90° geometry from each other in the plane of the spectrometer. The Near-Infrared (NIR) emission in the spectral range of 900-1100 nm for Yb 3+ and 1500-1600 nm for Er 3+ were integrated for the QY determination. All the spectral data collected were corrected for the spectral response of both the spectrometer and the integrating sphere. We calculated the absolute PLQY based on the following equation: where Ne and Na are the photons emitted and absorbed, respectively; Ls is the emission intensity, Er and Es are the intensities of the excitation light in the presence of the BaSO4 (reference) and Yb 3+ (Er 3+ ) doped Cs2NaInCl6 (sample) DPs, respectively. All the PLQYs for each sample were measured independently at least three times under identical conditions to yield the average value.
Characterization: Powder X-ray diffraction (XRD) patterns of the samples were collected with an X-ray diffractometer (MiniFlex 600, Rigaku) with Cu Kα1 radiation (λ = 0.154187 nm), operating at 40 kV and 40 mA. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements were performed at inductively coupled plasma atomic emission spectroscopy spectrometry (Ultima2, HORIBA Jobin Yvon). The diffuse reflectance spectra were recorded on a UV-vis-NIR spectrophotometer (Lambda950). Xray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250Xi X-ray Photoelectron Spectroscopy using a monochromatic Al Kα source (15 kV, 20 mA). The PL emission and PL excitation (PLE) spectra and PL decay curves were recorded on an Edinburgh FLS980 spectrofluorometer equipped with a continuous xenon lamp (450 W), a pulsed flash lamp and a 375 nm picosecond pulsed laser. Effective PL lifetimes (τeff) were calculated by: where I(t) denotes the PL intensity as a function of time t, and Imax is the maximum PL intensity.
Density functional theory (DFT) Calculation Details: The density functional theory (DFT) calculation was performed using the Vienna Ab-Initio Simulation Package. [1,2] The electron-ion interaction was described by projector augmentedwave (PAW) pseudopotentials. For the exchange and correlation functionals, we use the Perdew-Burke-Ernzerhof (PBE) version of the generalized gradient approximation (GGA) exchange-correlation. [3] The energy cutoff of 500 eV was used for the wave functions expansion. The energy and force converged to 1.0 × 10-5 eV atom -1 and 0.05 eV·Å -1 . The Brillouin zone integration was sampled with 2 × 2 × 2 k-grid mesh. The Heyd-Scuseria-Ernzerhof (HSE06) calculations were employed to obtain accurate bandgap. [4]          Orbital distribution profiles of Cs2AgInCl6:Yb 3+ showed that VBM was composed of a mixed configuration of Ag 4d and Cl 3p states, and CBM mainly consisted of In 5s states with minor contributions from Ag 4d and Cl 3p states. Such configuration benefited the formation of STE, which resulted from the Jahn-Teller distortion of the connected [AgCl6] 5--[InCl6] 3octahedron. For Cs2NaInCl6:Yb 3+ , VBM and CBM were essentially composed of Cl 3p states and In 5s states, respectively, which revealed that the orbitals were distributed over the whole supercell with little spatial overlap. Such poor spatial overlap led to the extremely weak edge-to-edge transition in this system. Figure S8. Vibronic PL spectra of Cs2NaInCl6:Yb 3+ at 10 K. The intense 2 F5/2 Γ8 →Γ6, Γ7, Γ8 2 F7/2, and 2 F5/2 Γ7 → Γ7 2 F7/2 transitions of Yb 3+ can be observed. Each transition shows rich vibronic emission (peaks with '*') deriving from vibrational modes of [YbCl6] 3-. [13]